Lithium-ion (Li+) secondary or rechargeable batteries are now the most widely used secondary battery systems for portable electronic devices. However, the growth in power and energy densities for lithium ion battery technology has stagnated in recent years as materials that exhibit both high capacities and safe, stable cycling have been slow to be developed. Much of the current research effort for the next generation of higher energy capacity materials has revolved around using small or nanoparticulate active material bound together with conductive agents and carbonaceous binders. These additives, though, dilute high capacity materials with a much lower or non-active material and limit the ultimate charge storage capacity of any battery incorporating such materials.
There is a current and growing need for higher power and energy density battery systems. The power requirements for small scale devices such as microelectromechanical systems (MEMS), small dimensional sensor systems, and integrated on-chip microelectronics exceed the power densities of current Li+ based energy storage systems. Power densities of at least 1 J/mm2 are desired for effective function for such systems, and current energy densities for Li+ thin film battery systems are about 0.02 J/mm2. Three dimensional architectures for battery design can improve the areal power density of Li+ secondary batteries by packing more active material per unit area without employing thicker films that are subject to excessive cycling fatigue. Three dimensional Lithium-ion battery architectures also increase lithium ion diffusion by maximizing the surface area to volume ratio and by reducing diffusion lengths.
The current state-of-the-art for anode electrodes in lithium ion batteries includes the use of high surface area carbon materials such as mesocarbon microbeads (MCMB). However, the capacity of any graphitic carbon, carbon black, or other carbonaceous material is limited to a theoretical maximum of 372 mAh/g and about 300 mAh/g in practice because carbon electrodes are usually formed of carbon particles mixed with a polymeric binder pressed together to form a bulk electrode. To store charge, Li+ intercalates between the planes of sp2 carbon atoms and this C—Li+—C moiety is reduced. In addition, the maximum number of Li+ that can be stored is one per every six carbon atoms—LiC6. While the capacity of graphitic carbon is not terribly high, the intercalation process preserves the crystal structure of the graphitic carbon, and so cycle life can be very good.
A more recent and promising option for anode materials is silicon (Si). In contrast to the intercalative charge storage observed in graphite, Si forms an alloy with lithium. Silicon-based negative electrodes are attractive because their high theoretical specific capacity of ˜4200 mAh/g, which far exceeds than that of carbon, and is second only to pure Li metal. This high capacity comes from the conversion of the Si electrode to a lithium silicide which at its maximum capacity has a formula of Li22Si6, storing over 25 times more Li per atom than carbon. The large influx of atoms upon alloying, however, causes volumetric expansion of the Si electrode by over 400%. This expansion causes strain in the electrode, and this strain is released by formation of fractures and eventual electrode failure. Repeated cycling between LixSiy and Si thus causes crumbling of the electrode and loss of interconnectivity of the material. For example, 1 zμm thick Si film anodes have short cyclability windows, with a precipitously capacity drop after only 20 cycles.
This strain from volumetric expansion has been mitigated, to some extent, by nanostructuring the anode material, providing room for material expansion. Nanostructuring Si films to reduce stress from lithium alloying has improved reversible capacity, though declines are still observed after 50 cycles. Going from a 2D Si electrode to 1D structures such as wires or an ensemble of 0D nanoparticles has been shown to further extend the cycle life of Si electrodes. Nanostructured materials for Li+ charge storage have demonstrated improved cycling lifetimes, larger specific capacities, and higher sustainable charge rates compared to bulk or thin film versions of the same materials. These improvements arise because nanoscale materials have free space to accommodate volume expansion and shorter diffusion lengths.
Thin Si films (400-500 nm) have also shown more stable charge storage capacity because fracturing of the film upon cycling can in some cases result in interconnected nanostructures with many of the desirable properties described above. Silicon electrodes have further been made by dispersing Si nanowires or nanoparticles in a carbon binder or matrix. These types of electrodes have shown good stability out to 50 cycles, but relatively poor specific capacity of 600 mAh/g due to the inert mass of matrix. For example, Maqasinski et al. have disclosed a Si-based lithium ion anode that uses annealed carbon-black dendritic particles that are coated with Si nanoparticles. Maqasinski et al., Nature Materials 9, 353-358 (March 2010). These nanoparticles are then assembled into larger rigid spheres with open internal channels. While such structures have shown some improved reversible capacities, the inclusion of carbon adversely affects batter performance.